Aspects of direct laser desorption/ionisation have been studied for three molecules, aminotriazole (positive ion), dinoterb and ioxynil (negative ion). The samples are deposited on metallic substrates, and a nitrogen laser is used for desorption/ionisation; ion yields are measured with a time-of-flight mass spectrometer. Previous work had shown that ion yields can strongly vary from one substrate to another, and that this variation does not reflect the (calculated) metal surface temperatures. New results obtained in this work indicate that the desorption/ionisation mechanism is linked to the physical state of the substrate surface. Copyright 1999 John Wiley & Sons, Ltd.